Which of the following methods provides the most rapid onset of action for drug delivery to the CNS?

Inhalation is indeed the method that provides the most rapid onset of action for drug delivery to the central nervous system (CNS). When substances are inhaled, they enter the lungs and subsequently diffuse directly into the bloodstream through the alveoli. This allows for very rapid absorption and distribution of the drug, often leading to effects being felt within minutes. The close proximity of the lungs to the circulatory system facilitates the quick transportation of the drug to the brain.

Oral administration, while common, generally takes much longer because the drug must pass through the digestive system, where it is metabolized before entering the bloodstream and reaching the CNS. Rectal administration can provide quicker effect than oral methods, but it still requires some time for the drug to be absorbed and metabolized.

Topical application usually targets local tissues and may not effectively penetrate systemic circulation to reach the brain quickly. Thus, when comparing these methods, inhalation stands out for its ability to deliver drugs with an immediate impact on the CNS.